Compacting of gas atomized metal powder to a part

ABSTRACT

A process for making full dense components of a carbon-containing steel, comprises the steps of: a) making a powder of the carbon-containing steel by gas atomization wherein the carbon content is low, less than 0.15 wt %, b) agglomerating the powder from step a) with at least one hydrocolloid and elemental carbon, c) compacting the agglomerated powder from step b) to a density of at least 80% of theoretical density, with the proviso that the compacted agglomerated powder still is porous allowing transport of gas to and from its interior, and d) sintering the compacted powder to a density of more than 98% of theoretical density, preferably more than 99% of theoretical density, wherein a gas comprising carbon is added during sintering and finally subjecting the component to HVC. Advantages include that it is possible to manufacture a dense component of powders which otherwise are difficult to compact.

TECHNICAL FIELD

The present invention relates generally to a process for compactingcarbon containing hardenable steels. It further relates to a partmanufactured by such a process.

BACKGROUND

Certain powder processes in the art use gas atomized powder ofpre-alloyed steel alloys unlike conventional powder technology and wherethe spherical gas atomized powder to obtain good green strengthagglomerated to a press powder before pressing/sintering. An example ofthe first mentioned process is described under the trade name Scanpac®MMS.

When using gas atomized powder of conventional carbon steel and otherhardenable steels, those are hardened during the atomization processitself because during the atomization process the cooling occurs sorapidly that the powder becomes hardened. The hardness is of coursedetermined by the alloy itself.

It is well known that the carbon content determines the hardness of atypical carbon steel and certain other alloys, e.g. martensitic,ferritic stainless chromium steel. Typically the carbon content ofcommercial carbon steel is from approximately 0.2 wt % (wt) and up toabout 1.3 wt %.

The hardenability of ferrous alloys, i.e. steels, is a function of thecarbon content and other alloying elements and the grain size of theaustenite. The hardenability of a ferrous alloy is measured by a Jominytest: a round metal bar of standard size (indicated in the top image) istransformed to 100% austenite through heat treatment, and is thenquenched on one end with room-temperature water. The cooling rate willbe highest at the end being quenched, and will decrease as distance fromthe end increases. Subsequent to cooling a flat surface is ground on thetest piece and the hardenability is then found by measuring the hardnessalong the bar. The farther away from the quenched end that the hardnessextends, the higher the hardenability.

Direct pressing of gas atomized powder of pre-alloyed steel alloys givesa hard powder with very low densities of the pressed component after theinitial pressing. This is obviously a disadvantage and adversely affectsthe subsequent process steps; sintering, re-compaction (restrike) andfinal sintering or capsular-free HIP, because the goal is to reach ashigh a density as possible. The goal is to reach a final density of over99% of the theoretical density.

One approach in the art is to soft anneal the metal powder before theagglomeration. This is however not straightforward, since the metalpowder will sinter to some extent during the soft annealing. After sucha soft annealing the pieces of sintered powder will have to be crushedand sieved to the correct particles size before use. This procedure isexpensive, complicated and time consuming.

Thus it is a problem in the prior art how to provide a method which inan easier, faster and less expensive way achieves a final density over99% (or at least over 98%) of the theoretical density for hardenablesteels, i.e. for alloys comprising carbon in particular carbon steel.

Summary

It is an object of the present invention to obviate at least some of thedisadvantages in the prior art and provide an improved method tomanufacture components of metal powder.

In a first aspect there is provided a process for making full densecomponents with a density of more than 98%, preferably more than 99% ofthe theoretical density of a material selected from the group consistingof a carbon-containing steel and a carbon containing alloy, said processcomprising the steps of:

-   a) making a powder of the carbon-containing steel by gas    atomization, wherein the carbon content is less than 0.15 w %,-   b) agglomerating the powder from step a) with at least one    hydrocolloid, wherein carbon in elemental form is added during the    agglomeration,-   c) compacting the agglomerated powder from step b) in a first    compacting step to a density of at least 80% of theoretical density,    with the proviso that the compacted agglomerated powder still is    porous allowing transport of gas to and from its interior,-   d) further compacting the compacted agglomerated powder from step c)    to a density of more than 98% of theoretical density using at least    sintering followed by HVC, high velocity compacting with a ram speed    exceeding 5 m/s, wherein a gas comprising carbon is added during    sintering.

In a second aspect there is provided a full dense component manufacturedaccording to the procedure above, with a density of more than 98% of thetheoretical density, wherein the component comprises a carbon-containingsteel.

Further aspects and embodiments are defined in the appended claims.

One advantage is that it is possible to manufacture components withtheoretical density (i.e. more than 98% of theoretical density,preferably more than 99% of theoretical density) of agglomeratedparticles of carbon steel which is otherwise difficult and/or at leastexpensive.

DETAILED DESCRIPTION

Before the invention is disclosed and described in detail, it is to beunderstood that this invention is not limited to particular compounds,configurations, method steps, substrates, and materials disclosed hereinas such compounds, configurations, method steps, substrates, andmaterials may vary somewhat. It is also to be understood that theterminology employed herein is used for the purpose of describingparticular embodiments only and is not intended to be limiting since thescope of the present invention is limited only by the appended claimsand equivalents thereof.

As used throughout the text a steel denotes an alloy of iron and otherelements, primarily carbon.

Theoretical density denotes the maximum density of a material which ispossible to achieve in theory, i.e. for a perfect material without anydefects.

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a”, “an” and “the” include plural referentsunless the context clearly dictates otherwise.

If nothing else is defined, any terms and scientific terminology usedherein are intended to have the meanings commonly understood by those ofskill in the art to which this invention pertains.

In a first aspect there is provided a process for making full densecomponents with a density of more than 98%, preferably more than 99% ofthe theoretical density of a material selected from the group consistingof a carbon-containing steel and a carbon containing alloy, said processcomprising the steps of:

-   a) making a powder of the carbon-containing steel by gas    atomization, wherein the carbon content is less than 0.15 w %,-   b) agglomerating the powder from step a) with at least one    hydrocolloid, wherein carbon in elemental form is added during the    agglomeration,-   c) compacting the agglomerated powder from step b) in a first    compacting step to a density of at least 80% of theoretical density,    with the proviso that the compacted agglomerated powder still is    porous allowing transport of gas to and from its interior,-   d) further compacting the compacted agglomerated powder from step c)    to a density of more than 98% of theoretical density using at least    sintering followed by HVC, high velocity compacting with a ram speed    exceeding 5 m/s, wherein a gas comprising carbon is added during    sintering.

In many embodiments the carbon-containing steel becomes hardenable afterthe steps a) to c) above, i.e. the type of alloy (steel) and the carboncontent after addition of the carbon in step b) is selected so that thematerial becomes possible to harden. In one embodiments thecarbon-containing steel is hardenable after step c). Thecarbon-containing steel prior to gas atomization in step a) is in oneembodiment treated so that the carbon content is reduced to a carboncontent less than 0.15 wt %. This reduction in carbon content makes itpossible to manufacture a powder using gas atomization which is easierthan many other methods, the gas atomization can thus be used even forsteels which have a high carbon content and are hardenable. Carbon inelemental form is added during the agglomeration in step b) so that thecarbon content is increased to the desired value. In one embodiment thecarbon content is selected so that the steel (or alloy) is hardenable.“Hardenability” or hardenable is the ability of an alloy comprising Fe-Cto be hardened by the forming martensite as a result of a given heattreatment.

All sintering atmospheres have a certain amount of oxygen. The amount ofoxygen is usually measured with a so called dew point meter whichmeasures the dew point. The dew point level is a measure of amount ofoxygen in the sintering atmosphere at a given temperature.

-   −5° C. corresponds to 0.4 % H₂O-   −10° C. corresponds to 0.27 % H₂O-   −20° C. corresponds to 0.1 % H₂O

The actual oxygen atom in the water combines with carbon and forms COwhich is a more stable compound.

If nothing is done to compensate and/or hinder this reaction the partwill be depleted of carbon and the material is not fulfilling itsanalysis, i.e. the desired amount of carbon in the material will not bereached.

Even if the furnace is started with a low dew point, when thetemperature in the furnace is raised, oxygen/H₂O is released from theinsulation, the powder, the binder itself and so on. A typical value ofdew point during sintering is −20° C.

Methane CH₄ is a stable compound up to high temperature but reacts withH₂O at high temperature and form CO and free hydrogen. You can thereforeuse a gas comprising carbon such as but not limited to methane as a“sacrifice” compound in order to prevent the loss of carbon in thepressed powder part during sintering in hydrogen.

By using a routine experiment the skilled person is able to decide theexact amount of CH₄ to be added in order to stabilize and create a“neutral” atmosphere during sintering to prevent the decarburization ofthe part. It is desired to create such an atmosphere in order to reachthe desired carbon content in the final product. The below example(s)give a guide for this, for instance example 6 below. The skilled personcan in the light of this description perform a routine experiment todetermine the exact amount of methane to be added. The exactconcentration needed depends on the furnace and on the amount ofmaterial in the furnace etc. A skilled person can repeat the experimentin example 6 below and similarly determine a suitable methaneconcentration to use.

The finished part has a carbon content higher than the initiallyprovided powder in step a) due to additions of carbon in elemental form(in step b) and the gas comprising carbon (in step c).

The gas comprising carbon is able to reach into the part after step c)since it is still porous. After the sintering in step d) the part is notporous anymore. After step c) the density is at least 80% of thetheoretical density but not too dense so that it is still porous with apercolated structure so that gas can pass to and from the inner regionsof the part. In one embodiment the density after step c) is in theinterval 80-90% of theoretical density. In an alternative embodiment thedensity after step c) is in the interval 85-90% of theoretical density.In yet another embodiment the density after step c) is in the interval80-92% of theoretical density. For most materials it is not suitable touse higher densities than 92% of theoretical density, since the powderwill not be porous anymore and would thus not allow transportation ofgases to and from the interior of the part. For some embodiments andmaterials it is also possible to compact to a density a bit lower than80% of the theoretical density in step c). Thus in one embodiment thefirst compacting step c) is to a density of at least 75% of thetheoretical density. The term “% of theoretical density” denotes thedensity of the part as a percentage of the theoretically highest densityfor the material. The density is calculated as weight per unit volume.

In one embodiment the carbon in elemental form added during theagglomeration in step b) is added in an amount corresponding to thedifference between the carbon content in the carbon-containing steel instep a) and the desired carbon content of the finished steel after stepd). It should be noted that the carbon content of the carbon-containingsteel in step a) could be very low.

In one embodiment the carbon in elemental form in step b) is added inthe form of graphite. In one embodiment the carbon in elemental form instep b) is added in the form of a carbon powder. In one embodiment thecarbon in elemental form in step b) is added in the form of particleswith an average diameter in the interval 0.1-10 μm. This is an advantagesince the small particles give a large surface which can react quickerwith the surrounding materia. Carbon in any form can be added as smallparticles, including but not limited to graphite.

In one embodiment the carbon in elemental form in step b) is added inthe form of a colloid of insoluble particles comprising carbon suspendedthroughout a liquid. The carbon in elemental form in a colloidalsuspension can be any type of particles comprising carbon, including butnot limited to elemental carbon and graphite. The addition of carbon incolloidal form has the advantage of ensuring an even distribution of thecarbon particles. The colloidal suspension of solid particles in aliquid is also referred to as a sol. Preferably the sol is stable andthe force originating from the Brownian motion of the particles is ofthe same order of magnitude or greater than the gravity force so thatthe sol does not settle by gravity. This ensures an even distribution ofthe carbon in the part. The stability of the sol is in one embodimentmaintained by adding a dispersing agent.

When sintering with carbon containing steels in reducing atmospheres itis important that the carbon content is controlled.

With the new process where pure carbon can be added in form of superfinegraphite it is clear that the carbon is even more easily removed due tothe fact that the carbon is in form of small particles on the existingpowder grains and therefore extremely easy to reduce as thepart/material is porous. The porosity is typically 10-15% (correspondingto a density of 85-90% of the theoretical density) allowing free flow ofthe gases/atmosphere at sintering.

In one embodiment the carbon-containing steel prior to gas atomizationin step a) is treated so that the carbon content is reduced to a carboncontent less than 0.10 wt %, preferably less than 0.050 wt %. The lowercarbon content prevents the metal particles from being hardened duringgas atomization.

The compacted agglomerated powder from step c) is sintered. A gascomprising carbon atoms is added during sintering in order to preventcarburizing or decarburization. The gas is in one embodiment methane. Itis also possible to conduct the method with other carbon bearing gasesincluding but not limited to propane and gasoline, but experimentsperformed so far show that methane gives the best accuracy and the bestresults, at least in the setup used in the experiments.

During step d) the compacted agglomerated powder from step c) is bothsintered and subjected to HVC and optionally further compacting steps.The HVC is a high velocity compacting with a ram speed of 5 m/s or more.In step d) it is conceived that the material is first sintered with thegas and then subjected to HVC. The compacting in step d) can thus beseparated in time, i.e. the material can first be sintered with the gasand after some time it can be subjected to HVC.

In one embodiment the carbon-containing steel prior to gas atomizationin step a) is treated with the process argon oxygen decarburization.Argon oxygen decarburization (AOD) is a process used for makingstainless steel and other high grade alloys with oxidizable elementssuch as chromium and aluminum. After initial melting the metal is thentransferred to an AOD vessel where it will be subjected to three stepsof refining; decarburization, reduction and desulphurization.

In one embodiment the carbon-containing steel prior to gas atomizationin step a) is treated in a vacuum furnace.

In a second aspect there is provided a full dense component manufacturedaccording to the procedure above, with a density of more than 98% of thetheoretical density, wherein the component comprises a carbon-containingsteel. In one embodiment the density is more than 99% of the theoreticaldensity.

In another aspect there is provided a process for making full densecomponents (i.e. a density of more than 98 wt %, preferably more than 99wt % of the theoretical density) of a carbon-containing steel, saidprocess comprising the steps of:

-   a) Making a powder of the carbon-containing steel by gas    atomization,-   b) Agglomerating the powder from step a) with at least one    hydrocolloid,-   c) compacting the agglomerated powder from step b) in a first    compacting step to a density of at least 80 wt % of theoretical    density,    wherein the carbon-containing steel is hardenable by the steps a)    to c) above, wherein the carbon-containing steel prior to gas    atomization in step a) is treated so that the carbon content is    reduced to a carbon content less than 0.15 wt % and that carbon in    elemental form is added during the agglomeration in step b).

In one embodiment the carbon in elemental form added during theagglomeration in step b) is added in an amount corresponding to thedifference between the carbon content in the carbon-containing steel instep a) and the desired carbon content of the finished steel.

In one embodiment the carbon in elemental form in step b) is added inthe form of graphite. In one embodiment the carbon in elemental form instep b) is added in the form of a carbon powder.

In one embodiment the carbon-containing steel prior to gas atomizationin step a) is treated so that the carbon content is reduced to a carboncontent less than 0.10 wt %, preferably less than 0.050 wt %.

In one embodiment the compacted agglomerated powder from step c) issintered. In one embodiment a gas comprising carbon atoms is addedduring sintering in order to prevent carburizing or decarburization. Inone embodiment the gas is methane.

In one embodiment the compacted agglomerated powder from step c) issubjected to at least one treatment selected from the group consistingof sintering, HVC (high velocity compaction, i.e. compaction with a ramspeed exceeding 5 m/s)

In one embodiment the carbon-containing steel prior to gas atomizationin step a) is treated with the process argon oxygen decarburization. Inone embodiment the carbon-containing steel prior to gas atomization instep a) is treated in a vacuum furnace.

All the described alternative embodiments above or parts of anembodiment can be freely combined without departing from the inventiveidea as long as the combination is not contradictory.

Other features and uses of the invention and their associated advantageswill be evident to a person skilled in the art upon reading thedescription and the examples.

It is to be understood that this invention is not limited to theparticular embodiments shown here. The embodiments are provided forillustrative purposes and are not intended to limit the scope of theinvention since the scope of the present invention is limited only bythe appended claims and equivalents thereof.

The following gives some examples of the new process.

EXAMPLES Example 1, Comparative

A 42CrMo4 steel type with a carbon content of about 0.4 wt % wasatomized and sieved to a powder with a maximum particle size of 150 μm.The powder was agglomerated and then pressed in a so-called HVC (HighVelocity Compaction) press. The density of the compacted component wasmeasured to be about 74% of the nominal density (theoretical density).This is a very low value. By comparison a standard stainless steel type316 L (1.4404) was pressed to a density of 90% with approximately thesame parameters. It is clear that even if the steel is optimallyprocessed further, it is very difficult, if not impossible, to reachfinal densities of 98-99% which is necessary so that the finishedcomponent shall get sufficient properties compared with the same forgedmaterials.

Example 2

There was another experiment with the same steel grade as in example 1.In this case the steel was passed through a so-called AOD (Argon OxygenDecarburizer) where the carbon content in the steel was reduced down toa carbon content of under 0.1 wt %, specifically 0.08 wt %. After AODtreatment the steel was atomized to powder as described above andagglomerated. After pressing with the same parameters there was obtaineda density of the pressed component of 86% of theoretical density. Toensure that the component after the pressing had a proper carbon contentthere was made an addition to the agglomeration mixture of finelydivided graphite, which compensated for the carbon removed with the AOD.The atmosphere in the subsequent sintering was hydrogen, with a smalladdition of methane with such a concentration that no decarburizationoccurred during sintering. The added graphite was absorbed by the fritand there was obtained an even distribution of the carbon contentthroughout the component. After sintering at 1250° C. and subsequentannealing and again HVC as well as final sintering there was obtained afinal density of 99.2% of theoretical density. The finished componentdisplayed normal structure and normal expected mechanical values.

Example 3

The same experiment as in example 2 was repeated, but the carbon wasreduced to 0.012 wt % carbon. At HVC pressing a density of 91% oftheoretical density was reached. The final density was in this case99.6% following the above steps.

Example 4, comparative

In another example, a steel normally used in the manufacture of ballbearings was manufactured. The steel is called 100Cr6 (1.3505). Thesteel has a carbon content of about 1 wt % and is relatively “hard” whenit is hardened. The density after direct compression was only 72% of thetheoretical density and thus the final density did not reach more than94% of theoretical density.

Example 5

The same process as in example 4 was repeated but where the carboncontent was reduced down to 0.009 wt % and made according to theinvention. It resulted in a density of the pressed component of 90%. Thefinal density was 99.2%, which gave good properties of the finishedcomponent. The final density was obtained after sintering with methaneand HVC.

Example 6

A powder with an analysis corresponding to a ball bearing steel 100 Cr6/EN 1.3505 but with a carbon content of 0.05% was agglomerated with ahydrocolloide. During the agglomeration carbon was added in form ofcolloidal graphite with a grain size of 1-3 μ. The amount of graphitewas so calculated that the final carbon content should be 1.05% which iswithin the range of the standard forged, wrought material.

The parts were pressed in a HVC press to 87% of theoretical density.

The parts were then heated in a batch furnace of type CM with a furnacespace of 0.4 m in cubic form to a temperature of 1170° C. Up to 400° C.where the binder/hydrocolloide was evaporized the heating speed was 100°C./hr. and after that 200° C./hr. The gas flow of pure hydrogen was 1.6m³/hr.

Methane was added through a flow meter at different rates. On the scale100 points meant a flow of Methane of 1.01×10⁻² m³ per hour. This meansthat about 1 wt % of the carrier gas was hydrogen.

In the first test the addition was 60 points on the scale. The finalcarbon analysis was 0.46 wt % carbon which meant that the addition ofCH4 was too low.

Second trial with 80 points of the scale showed a final carbon contentof 0.78 wt % C.

The next time 100 points gave a carbon content of 0.98 wt %

120 points on the scale gave a final carbon content of 1.13 wt %.

The above trial showed that with a scale factor for methane between100-110 point you got a carbon content within the limits of the standardfor the above material 100Cr 6 which is 0.93-1.05 wt % carbon.

The dew point which was measured during the whole cycle was at startwith unloaded furnace −60° C. but increased to approximately −20° C.during sintering.

Example 7

Similar trials as in example 6 with other carbon bearing gases aspropane, gasoline etc worked but did not give the same accuracy andstability as methane in this particular setup. It seems therefore as themethane has a stable behavior and can suitably be used industrially tocontrol the final carbon content when sintering above materials.

Sintering tests up to 1250° C. confirmed this behavior.

1. A process for making full dense components with a density of morethan 98%, of the theoretical density of a material selected from thegroup consisting of a carbon-containing steel and a carbon containingalloy, said process comprising the steps of: a) making a powder of thecarbon-containing steel by gas atomization, wherein the carbon contentis less than 0.15 wt %, b) agglomerating the powder from step a) with atleast one hydrocolloid, wherein carbon in elemental form is added in theform of a colloid of insoluble particles comprising carbon suspendedthroughout a liquid, during the agglomeration, c) compacting theagglomerated powder from step b) in a first compacting step to a densityof at least 80% of theoretical density, with the proviso that thecompacted agglomerated powder still is porous allowing transport of gasto and from its interior, d) further compacting the compactedagglomerated powder from step c) to a density of more than 98% oftheoretical density using at least sintering followed by HVC (highvelocity compacting) with a ram speed exceeding 5 m/s, wherein a gascomprising carbon is added during sintering.
 2. The process according toclaim 1, wherein the carbon in elemental form added during theagglomeration in step b) is added in an amount corresponding to thedifference between the carbon content in the carbon-containing steel instep a) and the desired carbon content of the finished steel after stepd).
 3. The process according to claim 1, wherein the carbon in elementalform in step b) is added in the form of graphite.
 4. The processaccording to claim 1, wherein the carbon in elemental form in step b) isadded in the form of a carbon powder.
 5. The process according to claim1, wherein the carbon in elemental form in step b) is added in the formof particles with an average size in the interval 0.1-10 μm.
 6. Theprocess according to claim 1, wherein the carbon-containing steel priorto gas atomization in step a) is treated so that the carbon content isreduced to a carbon content less than 0.10 wt %.
 7. The processaccording to claim 1, wherein the carbon-containing steel prior to gasatomization in step a) is treated so that the carbon content is reducedto a carbon content less than 0.050 wt %.
 8. The process according toclaim 1, wherein the carbon-containing steel prior to gas atomization instep a) is treated with argon oxygen decarburization.
 9. The processaccording to claim 1, wherein the carbon-containing steel prior to gasatomization in step a) is treated in a vacuum furnace.
 10. The processaccording to claim 1, wherein the carbon-containing steel is hardenableafter step c).
 11. The process according to claim 1, wherein the gascomprising carbon is methane.
 12. A full dense component manufacturedaccording to claim 1, with a density of more than 98% of the theoreticaldensity, wherein the component comprises a carbon-containing steel. 13.The full dense component according to claim 12, wherein the density ismore than 99% of the theoretical density.
 14. The process according toclaim 1, wherein in step d), the compacted agglomerated powder from stepd) is further compacted to a density of more than 99% of theoreticaldensity.